Heretofore, a fluorinated copolymer made of tetrafluoroethylene and propylene has been used as a rubber material excellent in heat resistance, voltage resistance, oxidation resistance and chemical resistance in a severe environment wherein usual rubber material is not durable.
It is known that in recent years, by taking advantage of the voltage resistance, oxidation resistance and chemical resistance of a fluorinated copolymer, the fluorinated copolymer has been used as a binder in a storage battery device such as a capacitor, a primary battery or a secondary battery for an electronic device or an electric car, for which a high output power, a high capacity and excellent cycle characteristics are required.
For example, Patent Document 1 discloses an example wherein a secondary battery was prepared by using, as a binder, a fluorinated copolymer wherein the molar ratio of structural units derived from tetrafluoroethylene/structural units derived from propylene was 56/44.